Corrosion prevention



Dec. 11, 1962 A. L. BISHoP coRRosIoN PREVENTION Filed Nov. 2, 1959 United States Patent Oiilice 3,ilb8,l70 Patented Dec. 11, 1962 3,068,179 CGRROSHON PREVENTHON Arthur L. Bishop, Anacortes, Wash., assigner to Texaco, inc., New York, NY., a corporation of Delaware Filed Nov. 2, 1959, Ser. No. 850,381 4 Claims. (Qi. 208-235) This invention relates to the prevention of corrosion in a demercaptanizing system. More particularly, it pertains to a method of preventing corrosion of ferrous metal apparatus engaged in the removal of mercaptans from a liquid petroleum fraction.

An obect of the invention is to reduce corrosion of ferrous metal apparatus employed in the solvent extraction of mercaptans from liquid petroleum fractions.

Another obect is to reduce corrosion of ferrous metal apparatus utilized in the regeneration o-f the mercaptan removing solvent.

Still another obect is to reduce corrosion of ferrous metal apparatus engaged in recycling the regenerated solvent for reuse.

Other objects of the invention will become apparent from the detailed disclosure of the invention hereinafter set forth.

The necessity of removing mercaptans from liquid petroleum fractions has been recognized for many years and their removal from refinery streams is universal. Mercaptans in petroleum hydrocarbon frac-tions are objectionable because they impart noxious odors thereto. In addition, mercaptans are antagonistic to tetrae-thyl lead, a well known octane appreciator for gasoline, and cause corrosion of ferrous metal apparatus coming in contact therewith.

Within the definition of Ithe term liquid petroleum fraction, l include the liquid hydrocrabon distillates of crude oil and the liquid hydrocarbon products derived from the thermal and catalytic cracking of said distiliates. In addition, I also include normally gaseous hydro-carbon distillates and cracking products which are maintained in the liquid state during the demercaptanizing operation by superatmospheric pressure. Specic examples of the petroleum fractions contemplated herein are gasoline, kerosine, lube oil, liqueiied aliphatic hydrocarbons of one to live carbon atoms and mixtures thereof.

One of the most widely employed means of removing mercaptans from petroleum fractions is extracting the mercaptans therefrom utilizing a water-sodium hydroxide or a Water-methanol-sodium hydroxide solution as the extractive solvent. The sodium hydroxide concentration in the extractive solvent is desirably maintained between labout 5 and 5() wt. percent with water or awter and methanol making up the remainder of the solvent. When a v-/atfr-methanol-caustic solution is employed, it is advantageous to maintain the water content at least equal to the sodium hydroxide content. The extraction is usually conducted from a moderate to an elevated temperature, eg., Sii-250 F., under atmospheric or superatmospheric pressure, e.g. up to 500 p.s.i.g. Superatmospheric pressure is generally employed when the extraction is operated at a temperature above the boiling point of the petroleum fraction to be demercaptanized and/or the ingredients of the extractive solution. The extraction is preferably accomplished by countercurrent contact of the petroleum fraction and caustic solution. In explanation of the chemical mechanics of the above described removal, the sodium hydroxide reacts with the mercaptans to form the corresponding sodium mercaptides which dissolve in the aqueous caustic solution to the substantial exclusion of the remainder of the petroleum fraction.

For reasons of economy the caustic extract solution after separation from the demercaptanized petroleum fraction is often recycled for further use and thus a regeneration step is necessary to remove the dissolved mercaptides therein. This regeneration is accomplished by decomposing the dissolved mercaptides to form mercaptans and sodium hydroxide and subsequently stripping out the regenerated mercaptans. The decomposition and stripping action is accomplished by bringing the mercaptide containing caustic solution in contact with steam.

in the regeneration procedure it is desirable to pass the steam and mercaptide containing caustic solution in counercurrent flow by introducing the heavier caustic solution in the top section of a regenerator at a temperature above about 212 F., e.g. 212-360" F. and introducing steam into the bottom section of the regenerator under atmospheric or superatmospheric pressure, e.g. 0-.60 psig.. By introducing the aqueous caustic at the top of the regenerator at a temperature above about 212 F.,

a minimum amount of condensation of the steam in the regenerator is insured. The steammay be supplied from an extraneous source. Alternatively, the steam can be obtained from the caustic solution itself utilizing a conventional reboiler maintained at a steam generating temperature and placed in communication with the aqueous caustic in the lower part of the regenerator. Whatever the steam source any water los-t by the aqueous caustic solution during the regeneration step may be replaced during the recycle of the treated caustic to the mercaptan absorption tower.

The demercaptanized aqueous caustic is withdrawn from the bottom of the regenerator and the mercaptans, Y

residual steam and methanol (if used) are withdrawn in gaseous form from the top of the regenerator. When methanol is employed as an ingredient in the extractive solvent it can be recovered by condensing the exit gases to form al two layer condensate. The methanol-Water layer is separated from the mercaptan layer by conventional means and the former can be sent to a fractionator to remove excess water if desired. In any case the methanol is then reintroduced into the regnerated caustic solution during the period of recycle of said solution to the initialrmercaptan removal stage.

The demercaptanized solvent is recycled to petroleum fraction treating stage of the process. Desirably when the sodium hydroxide concentration in the solvent falls below about 5 Wt. percent, either additional sodium hydroxide is added to the caustic solution in order to bring it up to the desired sodium hydroxide concentration or the spent caustic solution is replaced with fresh caustic solution.

The above deinercaptanizing and caustic regeneration process is highly corrosive tosteel, cast iron and other ferrous metals. Under the process conditions substances such as steam, mercaptans, sodium hydroxide and water all signiiicantly contribute to the corrosion problem. In the past, replacement of absorber towers, regeneration towers, caustic storage tanks and attendant ferrous metal.

thereby prevent corrosion throughout said system was needed.

in accordance with the above need, I have discovered that the incorporation of a member selected from the group consisting of trisodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate and orthophosphoric acid into the aqueous sodium hydroxide solvent prior to the introduction of said solvent into the above described mercaptan removal-solvent regenerationsolvent recycle system Substantially reduces the corrosion of the ferrous metal equipment involved. The phosphorus inhibitors contemplated herein have the distinct advantage of remaining in the caustic solution throughout the entire mercaptan removal-solvent regeneration-solvent recycle and suppressing all form of corrosion therein thereby rendering protection to the ferrous metal process equipment throughout. 'Ihe inhibitors of the invention being readily soluble in Water may be dissolved in the aqueous caustic by mere mixing, e.g. with a merchanical stirrer or compressed air. The concentration of the phosphorus inhibitors in the caustic solution is desirably maintained between about .0l and 10 wt. percent, more preferably between .05 and wt. percent. Trisodium phosphate is the preferred phosphorus inhibitor. However, the other phosphorus compounds contemplated herein are equally effective since they are converted into trisodium phosphate by the aqueous caustic solution.

The following example further illustrates my invention:

Example 1 Referring to the drawing a liqueed butylene feed stock under a pressure of 80 p.s.i;g. and at a temperature of 83 F. was fed through line 1 into the bottom section of absorber 2 which contains 23 shower decks at a rate of 170 barrels per hour (b.p.h.). The butylene feed stock was derived from the catalytic cracking of gas oil and contained an average of 0.13 pound of mercaptan sulfur per barrel thereof. The feed stock was of the following hydrocarbon composition.

As the butylene feed stock was introduced into the bottom section ofabsorber 2, inhibited aqueous sodium hydroxide was introduced into the top section of absorber 2 through line 3 at a temperature of between 110 and 120 F. and at a rate of 42 b.p'.h. The inhibited aqueous caustic solution was prepared by introducing 150 barrels of water into storage and mixing tank 4, adding 30 pounds of sodium hydroxide per barrel of water and 100 pounds of trisodium phosphate. The mixture was agitated by bubbling compressed air into the bottom of tank 4 through line 5 and open valve 6. When solution was accomplished, valve 6 was closed. These ingredient proportions -gave an inhibited aqueous caustic solution containing 7.9 wt. percent sodium hydroxide and 0.18 Wt. percent trisodium phosphate. Upon introduction of lthe inhibited sodium hydroxide solution into absorber 2, the solution gravitated in countercurrent flow through the rising mercaptan containing butylene feed stock converting the mercaptans into their sodium mercaptide salts and absorbing the salts therein. The demercaptanized hydrocarbon stream entraining some aqueous caustic passed overhead through line 7 into knockout drum 8. The heavier caustic solution was withdrawn through line 9 to storage tank 4. The desulfurized liquefied butylene feed stock was withdrawn from tank 8 through line 10 for conversion into isooctane by alkylation units (not shown). The main body of aqueous sodium hydroxide containing the dissolved mercaptides was discharged from absorber 2 through line 11 to heat exchanger 12 and thence through line 13 to preheater 14. The aqueous caustic was heated by the exchanger 12 and preheater 14 combination to la temperature of 245 F. The heat source of heat exchanger 12 was derived from the passage of the regenerated inhibited aqueous caustic at F. and under a pressure of 9 p.s.i.g from the bottom section of regenerator tower 15 through line 16 into the tubing of exchanger 12. The heat of preheater 14 was derived from the introduction of steam at 210 p.s.i.g. over the tubing of the preheater via line 19. The residual and condensed steam were withdrawn from preheater 14 via line 17.

The heated aqueous caustic absonber bottoms were withdrawn from preheater 14 via line 18 and introduced into the upper section of regenerator tower 15 containing l0 bubble cap trays. The heavy caustic gravitated down through regenerator 1S. Simultaneously, steam under a pressure of 15 p.s.i.g. (250 F.) at a rate of 3100 pounds per hour was introduced through line 26 into the bottom section of regenerator 15. The steam and mercaptide containing aqueous solution passed in countercurrent ilow with the steam decomposing the mercaptide into the corresponding rnercaptans and stripping out the mercaptans from the aqueous caustic. The stripped out mercaptans and steam were withdrawn overhead from regenerator 15 through line 20.

The essentially mercaptan-free regenerated aqueous caustic was withdrawn from the bottom section of regenerator 15 through line 16 at a temperature of 156 F. under a pressure of 9 p.s.i.g. The withdrawn aqueous caustic solution was cooled to 118F. by passing said solution through the tubing heat exchanger 12 and over the tubing heat exchanger 22 via lines 16 and 21. Heat exchanger 22 was cooled by passing tap water therethrough via lines 23 and 24. The cooled regenerated aqueous caustic was withdrawn from heat exchanger 22 through line 25 into caustic storage and mixing tank 4 and the stored aqueous caustic was recycled from tank 4 through line 3 to absorber 2, as needed.

In addition to the above described process apparatus of Example 1 there were pumps, valves as well as temperature, pressure, flow measuring and regulating devices. Since the operation and mode of utilizing these devices in the above process would be obvious and their use merely incidental to the invention, they have not been specifically described.

The apparatus employed in the above described process was composed of carbon steel. After over a year of operation there was no replacement of the apparatus of any part thereof for reason of corrosion. Furthermore, a visual inspection of the apparatus for the same period of time found no evidence of corrosion except for some minor pitting in the bottom of regenerator 15. In contrast prior to the use of trisodium phosphate in the process of Example 1 the surfaces of the carbon steel apparatus after a year of exposure to the process ingredients were found to be severely pitted. Furthermore, the tubing in the heat exchangers and preheater, the bubble cap trays of the regenerator tower, and the shower decks of the absorber tower required replacement for reason of corrosion on an average of twice a year.

All percentages,.parts and ratios herebefore and hereafter recited are based on weight unless otherwise stated.

Obviously, many modifications and variations of the invention as hereinbefore set forth, may be made without departing from the spirit and scope thereof, and therefore, only such limitations should be imposed as are indicated in the appended claims.

I claim:

1. A method of inhibiting the corrosion of ferrous metal contacting the fluids of a mercaptan removal system consisting essentially of in the presence of said metal introducing into an between 5 and 50 wt. percent aqueous sodium hydroxide solution a member selected from the group consisting of trisodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate and orthophosphoric acid in an amount to insure said member concentration in said solution is betweenl and 10 wt.

percent, intimately contacting a mercaptan containing liquid petroleum fraction with said solution thereby forming an aqueous phase of a sodium mercaptide content and a petroleum fraction phase of reduced mercaptan content, separating said aqueous phase from said petroleum fraction phase, contacting said aqueous phase with steam thereby converting said mercaptide in said aqueous phase into said mercaptan and thereby stripping said mercaptan therefrom, separating said mercaptan and residual steam from said aqueous phase and subsequently recycling said aqueous phase of reduced mercaptide content .for intimate contacting with additional amounts of said petroleum fraction.

2. A method in accordance with claim 1 wherein said liquid petroleum fraction is a liquefied hydrocarbon derived from the catalytic cracking of gas oils and said member is trisodium phosphate.

3. A method in accordance with claim 1 wherein said solution contains methanol and said methanol is removed from said aqueous phase by said stripping and wherein the removed methanol is recovered and added to said aqueous phase of reduced mercaptide content.

4. A continuous method of inhibiting the corrosion of ferrous metal contacting the fluids of a mercaptan removal system consisting essentially of in the presence of said metal mixing trisodium phosphate with a 5 to 50 wt. percent aqueous sodium hydroxide solution in an amount to insure a phosphate concentration in said solu tion between .05 and 5 wt. percent, passing a mercaptan containing liquid petroleum fraction in countercurrent flow to said solution thereby forming an aqueous phase of sodium hydroxide, trisodium phosphate and sodium mercaptide and a liquid petroleum fraction phase of reduced mercaptan content, separating said aqueous phase from said liquid petroleum fraction phase, contacting the separated aqueous phase with steam in countercurrent flow thereby converting said mercaptide into said mercaptan and thereby stripping o said mercaptan from said separated aqueous phase, separating residual steam and reconverted mercaptan from the demercaptanized aqueous phase and recycling said demercaptanized aqueous phase for countercurrent contact with additional mercaptan containing liquid petroleum fraction.

References Cited in the le of this patent UNITED STATES PATENTS UNITED STATES PATETIT oFFTCE CERTIFICATE 0F CORRECTION Patent NGI, 3,068, 170 December 1l,Y 1962 Arthur L, Bishop It is hereby certified that error appears in the above numbered patent requiring correction and that the vsaid Letters Patent should read as corrected below.

Column I, linel 19 for "obect" read mobject line 5l17 for "'awter" read water column lI line 70 for A"an between" read a between about e-f.

Signed and sealed this 16th day of July 1963 (SEAL) Attesa ERNEST w. SWTDER DAVID L- LADD Attesting Officer Commissioner of Patents 

1. A METHOD OF INHIBITING THE CORROSION OF FERROUS METAL CONTACTING THE FLUIDS OF A MERCAPTAN REMOVAL SYSTEM CONSISTING ESSENTIALLY OF IN THE PRESENCE OF SAID METAL INTRODUCING INTO AN BETWEEN 5 AND 50 WT. PERCENT AQUEOUS SODIUM HYDROXIDE SOLUTION A MEMBER SELECTED FROM THE GROUP CONSISTING OF TRISODIUM PHOSPHATE, DISODIUM HYDROGEN PHOSPHATE, SODIUM DIHYDROGEN PHOSPHATE AND ORTHOPHOSPHORIC ACID AND AN AMOUNT TO INSURE SAID MEMBER CONCENTRATION IN SAID SOLUTION IS BETWEEN .01 AND 10 WT. PERCENT, INTIMATELY CONTACTING A MERCAPTAN CONTAINING LIQUID PETROLEUM FRACTION WITH SAID SOLUTION THEREBY FORMING A AQUEOUS PHASE OF A SODIUM MERCAPTIDE CONTENT AND A PETROLEUM FRACTION PHASE OF REDUCED MERCAPTAN CONTENT, SEPARATING SAID AQUEOUS PHASE FROM SAID PETROLEUM FRACTION PHASE, CONTACTING SAID AQUEOUS PHASE WITH STEAM THEREBY CONVERTING SAID MERCAPTIDE IN SAID AQUEOUS PHASE INTO SAID MERCAPTAN AND THEREBY STRIPPING SAID 